linux-kernel - Re: [PATCH] replace kmem_cache_alloc with kmem_cache_zalloc to remove some following zero initializations.

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Message-ID: <91b13c310707130138p6cffd0b4va34eec320d66ab44@mail.gmail.com>
Date:	Fri, 13 Jul 2007 16:38:44 +0800
From:	"rae l" <crquan@...il.com>
To:	"Kirill Korotaev" <dev@...ru>
Cc:	trivial@...nel.org, Denis <cr_quan@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] replace kmem_cache_alloc with kmem_cache_zalloc to remove some following zero initializations.

On 7/13/07, Kirill Korotaev <dev@...ru> wrote:
> This doesn't look worth zeroing half of the struct
> when it is initialized to non-zeros then.
But why?

My reason to think it's better and faster is that:
1. the code will be shorter if it calls zalloc and then removes the
NULL and zero initilization;
2. in the assembly code objdumped, many mov operations reduced, such as:
    movl $0,0x40(%ebp)
    ...
    this style of zero initialization occupies 7 bytes per line
(i386), and then multiply 7 lines,

3. the only change is that calls to kmem_cache_zalloc other than
kmem_cache_alloc, it's just an extra memset is called, as we all know
the memset implimentation is string operation, that's rather fast.

>
> Denis Cheng wrote:
> >>From 4d87e14b67890f06885a76b5792ca034de2e9d06 Mon Sep 17 00:00:00 2001
> > From: Denis Cheng <crquan@...il.com>
> > Date: Thu, 12 Jul 2007 11:53:58 +0800
> > Subject: [PATCH] replace kmem_cache_alloc with kmem_cache_zalloc to
> > remove some following zero initializations.
> >
> > Signed-off-by: Denis Cheng <crquan@...il.com>
> > ---
> >  fs/dcache.c |   12 ++----------
> >  1 files changed, 2 insertions(+), 10 deletions(-)
> >
> > diff --git a/fs/dcache.c b/fs/dcache.c
> > index 0e73aa0..8c559b2 100644
> > --- a/fs/dcache.c
> > +++ b/fs/dcache.c
> > @@ -898,7 +898,7 @@ struct dentry *d_alloc(struct dentry * parent, const
> > struct qstr *name)
> >       struct dentry *dentry;
> >       char *dname;
> >
> > -     dentry = kmem_cache_alloc(dentry_cache, GFP_KERNEL);
> > +     dentry = kmem_cache_zalloc(dentry_cache, GFP_KERNEL);
> >       if (!dentry)
> >               return NULL;
> >
> > @@ -921,15 +921,7 @@ struct dentry *d_alloc(struct dentry * parent,
> > const struct qstr *name)
> >       atomic_set(&dentry->d_count, 1);
> >       dentry->d_flags = DCACHE_UNHASHED;
> >       spin_lock_init(&dentry->d_lock);
> > -     dentry->d_inode = NULL;
> > -     dentry->d_parent = NULL;
> > -     dentry->d_sb = NULL;
> > -     dentry->d_op = NULL;
> > -     dentry->d_fsdata = NULL;
> > -     dentry->d_mounted = 0;
> > -#ifdef CONFIG_PROFILING
> > -     dentry->d_cookie = NULL;
> > -#endif
> > +
> >       INIT_HLIST_NODE(&dentry->d_hash);
> >       INIT_LIST_HEAD(&dentry->d_lru);
> >       INIT_LIST_HEAD(&dentry->d_subdirs);
>
>


-- 
Denis Cheng
Linux Application Developer

"One of my most productive days was throwing away 1000 lines of code."
 - Ken Thompson.
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